1. Field of the Invention
The present invention is directed to polymeric materials comprising biodegradable copolymers that exhibit shape-memory effects and implantable devices (e.g., drug-delivery stents) formed of such polymeric materials.
2. Description of the State of the Art
An implantable device that exists in a compressed size and then changes to a shape suitable for a particular medical need upon its deployment would have numerous medical applications (e.g., as a stent). Such a device should exhibit mechanical properties (e.g., strength, rigidity, toughness and flexibility) appropriate for its medical functions. To prevent inflammatory responses to the device and side reactions caused by harmful breakdown products of the device, the device should also biodegrade into biocompatible substances after it accomplishes its medical functions.
As an example, stents are often used in the treatment of atherosclerotic stenosis in blood vessels. To reduce the occlusion of the artery by the collapse of arterial lining and to reduce the chance of thrombosis and restenosis following angioplasty in the vascular system, a stent can be implanted in the lumen to reinforce the blood vessel and maintain the vascular patency. It would be useful if stents are made of a material that allows the stents to exist in a compressed shape, so that they can be inserted through small vessels via catheters, and then allows the stents to self-expand to the desired diameter once they are deployed at the treatment site. To act effectively as a scaffolding, i.e., physically holding open and, if desired, expanding the wall of a passageway, stents must possess good strength, rigidity, toughness and flexibility.
Stents are also used as a vehicle for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site, thereby possibly avoiding side effects associated with systemic administration of such substance. One method of medicating stents involves the use of a polymeric carrier coated onto the surface of a stent, where a therapeutic substance is impregnated in the polymeric carrier.
Late stent thrombosis has emerged as a concern for drug-delivery stents. The incidence of late stent thrombosis appears to be higher with drug-delivery stents than with the corresponding bare metal stents. One potential cause of late thrombosis with drug-delivery stents is a chronic inflammatory or hypersensitivity response to the polymeric coating on the stent.
The present invention is directed to biodegradable polymeric materials that exhibit shape-memory effects. Used for making implantable devices (e.g., stents), the polymeric materials enable the devices to adopt the appropriate shape upon deployment in the body, perform their mechanical and therapeutic functions more effectively, and avoid adverse effects such as late stent thrombosis.